1. Field of the Invention
The invention relates to drill pipe for subterranean wells, and the like, having tool joints secured to well drill pipe sections.
2. Brief Description of the Prior Art
Drill pipe strings, which comprise multiple drill pipe string sections threadably connectable to one another, are used to drill subterranean wells. Drill pipe string sections comprise tool joints and drill pipe sections, typically welded to each other. When drill pipe is used to drill subterranean wells, the drill pipe sections are exposed to bending, torsional, and other stresses. Such stresses are primarily due to hole curvatures extending through the entire length of the drilled hole and to the rotating motion of the drill pipe string. Such stresses may cause fatigue of the drill pipe sections due to fluctuating reversed bending stress which is imposed during rotation of the drill pipe string. During these fluctuating reversed bending stresses, this portion of the pipe is alternately subjected to pressure, tensile, and twisting or torsional forces as the drill pipe rotates. If the drill pipe falls by such resulting fatigue, the location of the fatigue point is oftentimes approximate the area of securement thereof to the tool joint, i.e., from about 1 to about 5 feet from the point of securement of the top end of the pipe to the tool joint. The stress in this portion of the drill pipe is usually considerably higher than the stress that is imposed on the remaining portions of the drill pipe string section.
If pipe fails by fatigue, the fatigue often originates in a slip mark. Slip marks occur at the end of the drill pipe attached to the tool joint, because rotatory slips are used to support the drill pipe string during the make and break cycles while drilling and tripping out of a well hole occur. These slips act as wedges that hold the entire weight of the drill pipe string. The portion of the slips that touch the pipe have teeth that can dig into or notch the pipe sections. These notches can act as stress risers that can act as a site for premature fatigue crack initiation and propagation. The above-mentioned bending stresses can initiate these fatigue cracks, thus causing the pipe to fail.
Applicant is aware of the following prior art which is addressed to similar problems of stress on subterranean well drill pipe string sections, but which does not anticipate or render obvious the present invention: (1) U.S. Pat. No. 2,676,820, issued Apr. 27, 1954, and entitled "Drill Collar"; (2) U.S. Pat. No. 3,554,307, issued Jan. 12, 1971, and entitled "Turbulent Flow Drill Collar"; (3) U.S. Pat. No. 3,666,022, issued May 30, 1972, and entitled "Striking Bar"; (4) U.S. Pat. No. 3,730,286, issued May 1, 1973, and entitled "Apparatus for Improving Rotary Drilling Operations"; (5) U.S. Pat. No. 4,811,800, issued Mar. 14, 1989, and entitled "Flexible Drill String Member Especially for Use in Directional Drilling"; and (6) U.S. Pat. No. 5,040,622, issued Aug. 20, 1991, and entitled "Variable Depth Grooved Drill String Member."
In the prior art, a portion of drill pipe consisting of a thicker, more fatigue-resistant material may be used to resist these higher stresses to diminish the tendency of the pipe to fall in the area of securement by slips more often than in other areas of the drill pipe string section. However, the use of the thicker portion of drill pipe section is used to strengthen directly the portion of drill pipe subject to higher stresses, rather than to redistribute part of the stress to stronger areas of the drill pipe string section or to areas more able to absorb or tolerate such stresses.
The present invention addresses some of the deficiencies of the prior art pipe string sections that were more prone to fail in the area of securement by slips more often than in other areas of the pipe, by providing a helical groove machined in the inner or outer surface of the drill pipe string section. Bending and other stresses to which these portions of the pipe sections are subjected are thereby redistributed to and partially absorbed by the portion of the drill pipe string section that contains the helical groove (typically the neck of the tool joint), since the presence of the helical groove makes this portion of the drill pipe string section relatively more resistant to failure due to fatigue, bending, and other stresses by allowing the string section region near the groove to flex more before failure. Further, the helical groove is not in alignment with planes perpendicular to the string section's axis, which tends to reduce the magnitude of the fluctuating stress that occurs when reverse bending stresses are imposed during drilling operations and rotation of the drill pipe. Because of the helical groove, therefore, the portion of the drill pipe string section containing the groove bends more easily than non-grooved portions of the tool joint and pipe section, thereby drawing stress away from these areas and to the grooved region of the drill pipe string section, thus making these other high-stress areas less prone to failure due to fatigue caused by stress. The portion of the string section which contains the groove is correspondingly more able to absorb these redistributed pressures and stresses because of "shock-absorber"-like action of the groove. The helical angle of the groove also tends to reduce fluctuating stresses as discussed hereinabove, since all portions of the groove do not lie on a common circumference of a rotating drill pipe string section. Additionally, the helical groove may be machined into stronger or more resilient portions of the pipe string section, such as the tool joint neck used in the preferred embodiment herein, to redistribute stresses from relatively weaker or more rigid portions of the string section to a stronger and more resilient portion of the string section, which is thus more able to tolerate stresses.